I already toasted one motor since i installed the lt-8 a few months ago. I just realized today when i was outside trying to understand the software that it shows 5in of vacuum with the motor not even in the car, WTF? Is it b/c of my elevation? I was under the impression that it should be at 0. If this is how it is supposed to be then wouldnt the turbo making more boost than the microtech was seeing and cause problems? I have a greddy electronic boost gauge that is coming off the same nipple that starts right at 0. Thanks for any info.

Derek

 

 

I forgot to update this thread. I emailed the techs from Microtech and apparently microtechs do not recalibrate to 0 depending on the altitude. I'm in Salt Lake City (4,500 ft?). Thats fine you just have to know that the boost reading on the microtech will always be off a few lbs. Another one of the guys in my area is running a microtech lt-8 as well and his shows the same readings. I asked if this would make the basemap dangerous and he said it wouldnt but i've thought about it quite a bit and i think it would. If the computer thinks i'm running 6 psi and the manifold is actually seeing 8 or 9 i'm going to be running pretty lean on the basemap. Of course i've learned my lesson and the car is going to be trailered to be tuned with a wb and an egt as soon as i throw a new motor in.

 

at that altitude the normal everyday pressure is different. like there is less pressure in the atmosphere and that is what the microtech is reading. when ur turbo is hiting 6 psi then ur microtech is reading 6psi. its hard to explain but theres nothing ot worry about, its fine.

 

I understand that the turbo is actually making more boost than the motor is seeing. The part i dont understand is why does my greddy boost gauge read different than the microtech. The greddy gauge is electronic.

 

what does your greddy guage read and what does ur microtech read? did you ever think that the greddy might recalibrate itslef since its electronic to the new atmosphere pressure since your higher?

 

My motor isn't in the car at this point and it only ran for about a week and that was a few months ago so i dont have any numbers. I'm pretty sure the boost gauge is recalibrating itself to zero every time i turn the key on. As soon as i talked to the guy from microtech it all made sense. I was concerned that my microtech was giving false readings. Correct me if i'm wrong, but the numbers from the boost gauge is going to show how much boost the turbo is actually making while the numbers on the microtech will show the amount of boost the motor is actually seeing? If i'm looking to run about 10-11 psi which should i go by? Thanks.

 

double

 

where is the boost guage reading boost from and where is the microtech reading boost from?

 

The standard pressure lapse rate at lower altitudes is -1"Hg per 1000'. Albuquerque is around 5000' altitude. Therefore, your outside air pressure should be about 5"Hg less than standard sea level pressure. Therefore, if the Microtech assumes that standard sea level pressure is 0"Hg, it will read 5"Hg less than that at your altitude, which would give an indication of -5"Hg. Sound familiar?

Just keep in mind that the Microtech boost indication is going to have an error of -1"Hg or -1/2psi per 1000' of altitude.

Not necessarily. The boost gauge shows the amount of pressure over ambient at the point at which the gauge probe is inserted. Therefore, it will only show the actual boost of the turbo if the gauge probe is inserted at the output of the compressor. Most people place the gauge probe in the intake manifold, which will cause the gauge to read manifold pressure, which is the amount of boost going into the engine (more or less).

Atmospheric Pressure + Gauge Pressure = Absolute Pressure

The MAP (Manifold Absolute Pressure) sensor reads Absolute Pressure. The Boost Gauge reads Gauge Pressure.

For more information on gauge vs. absolute pressure and other EMS-related material, see here:
http://www.scuderiaciriani.com/rx7/A...asics_V1.3.pdf

 

Was the MT calibrated at sea level? Or can we assume that some of this correction is already accounted for?

IOW -- if the MT was calibrated to zero at 1000', then you need to subtract 1000' from your altitude before making the correction?

or am I missing something? and/or making it too complicated?

 

It gets kinda confusing but Manifold absolute pressure is different than what an actual gauge reads. A gauge usually reads gauge pressure. MAP is the absolute pressure. If you read the info in the AEM user guide thats posted above it makes a lot more sense. Sorry for making it even more confusing than it was.

 

It gets kinda confusing but Manifold absolute pressure is different than what an actual gauge reads. A gauge usually reads gauge pressure. MAP is the absolute pressure. If you read the info in the AEM user guide thats posted above it makes a lot more sense. Sorry for making it even more confusing than it was.

 

Uh... your air is thinner than what it is at sea level. The Microtech sees this and corrects for it. If it read 0, it would be wrong. The computer needs to know the true density of the air. You're not at sea level, you don't get 0 as a reading at atmosphere.

Evil Aviator explained it perfectly, if you take time to read what he said you'll get it.

 

I understand it completly. I know the air is thinner here and knew that from day one. I was responding to TRAY's post. If you read my post you would have seen that i said "if you read the info on the AEM user guide posted above it makes a lot more sense". My biggest concern was why i was seeing 0 on my boost gauge and vacuum on the microtech. Evil Aviator did explain it very well. Thank you.